In 500pts I've learned not to bother with them due to their point cost and needing numbers (of warriors) to give my opponent something to fight against.
In higher point games, 750pts+, I've fielded them singly and in pairs. With various
results. As stated before, understand what you are facing...understand what has to be rolled to make the ballista effective and in that way you can decide if taking one is worth the points spenditure. The main reason I have taken a ballista is that it forces your opponent to take it in account in every move. Make sure you have at least 3 crew members, that way you can move it around the battlefield when you need to at 5" (pts wise it adds 8pts to the ballista). They will either: dedicate troops to take it out, holding back till the dwarves engage in H-t-H, or spreading out so that the 6" scatter doesn't kill anyone of their troops. Hero's and Hero Caster can negate the benefits of having a ballista..but..it will take them a few turns to do so. Giving you time to secure defensible areas or even setting up to take out said Hero's.
Be ready to lose it early on. Either your opponent will have sent enough force to neutralize it or whittle the crew down to 1...again neutralizing it. If you do take one..or two..make sure you have a screen of warriors handy for the pesky enemy units sent to tackle it.
For me, I like to change up what I bring and to have fun when I play. Sometimes that means enjoying the
randomness of a scatter dice roll or "seeing" your enemy flying through the air at the business end of a stone bullet!!
Best of luck -Scar